(1) Field of the Invention
This invention relates to a continuous production process of a styrene-base resin, especially, rubber-modified styrene-acrylonitrile resin having improved moldability.
(2) Description of the Prior Art
Rubber-modified styrene-acrylonitrile resins (ABS) are used for a wide variety of applications because their properties such as impact strength, stiffness, moldability, dimensional stability and the like are well-balanced. Reflecting the expansion of their application fields in recent years, there is a tendency toward greater, more complex and/or thinner molded articles. In order to meet such a recent trend, it is desired to improve the moldability of resins. For this purpose, it is widely practised to improve the moldability by adding higher fatty acid amides and/or metallic soaps. Turning to production processes of resins, batchwise polymerization processes relying upon emulsion polymerization or suspension polymerization have heretofore been employed. In each of such processes, a solid resin formed in accordance with batchwise polymerization is subjected to two additional steps prior to obtaining the final resin product, namely, the solid resin is blended with one or more of the above-mentioned additives and the resultant resin mixture is kneaded in an extruder or the like (see, for example, Japanese Patent Publication No. 23182/1973). Each of such processes however requires a great deal of labor and substantial facilities. There is thus a strong demand for the development of an improved method for the addition of such additives.
Incorporation of a large amount of such a moldability-improving additive in a resin by such a method will usually lead to insufficient feeding, reduced productivity and/or insufficient mixing if a conventional extruder is employed. To cope with this problem, it will be necessary, for example, to extrude the resin and additive twice.
Furthermore, a resin obtained by emulsion polymerization or the like contains, as impurities, residues of emulsifier, coagulant and the like. When such a moldability-improving additive is added to such a resin, the hue of the resin will be rendered poorer due to its heat history or the like and the resulting molded article will have inferior thermal discoloration resistance. An additional incorporation of a stabilizer such as an organic phosphorous acid ester is thus needed (see, for example, Japanese Patent Publication No. 45192/1972).
In the production of large and thin-walled molded articles having complex configurations, they may be cracked at portions corresponding to ejector pins or at corner areas upon taking them out of their corresponding molds, thereby failing to provide good molded articles. To cope with this problem, the mold release properties of such molded articles from their corresponding molds are improved by coating a silicone-type mold release agent or the like on the inner surfaces of the molds or adding a mold release agent such as a higher fatty acid or a metallic soap thereof. Even when such a silicon-type mold release agent or the like is coated on the inner surface of a mold, its effects last shortly. It is therefore necessary to coat such a mold release agent prior to each molding shot or even in the most successful case, still as often as every 10th molding shot. Use of such a mold release agent is hence time-consuming and moreover, it cannot be considered to be an advantageous method from the economical viewpoint. When higher fatty acids are used as mold release agents on the other hand, mold release properties can certainly be improved. However, use of higher fatty acids results in corrosion of extruders and molding machines, adhesion of deposit to die nozzles or molds upon extrusion or molding, sacrificed hues of products, thermal discoloration, etc. Therefore, it cannot be considered to be a preferred method.
On the other hand, metallic soaps of higher fatty acids, especially, their calcium soaps, magnesium soaps and the like have viscosities as high as several hundred thousands poises in molten state. It is therefore difficult to add them in molten state by usual pumps.